Helped: 68 times Age: 41 Joined: 16 Jan 2005 Posts: 14327 Location: Jakarta-Purwokerto PP
Posted: Sat Oct 01, 2005 5:33 pm Wireless how to....introducing,specification abd instalation
Introduction
Installing a wireless network requires even more planning than a wired one. There are additional factors such as Radio Frequency (RF) link planning, site preparation and installation of outdoor components such as radios, antennas, lightning protection devices as well as cabling which need to be considered. The installer also needs to be aware of zoning laws in the country or state.
Outdoor wireless equipments can be used under various schemes or topologies. The placement of these equipment depends on various factors such as distance, required throughput, Line of Sight (LOS), Fresnel Zone Interference, Fade margin etc. Thus a link can be calculated based on these factors which is able to give the link stability and throughput.
The above diagram shows an example of a basic Wireless WAN topology with Point-to-Point (PtP) using the smartBridges airHaul™ Nexus and Point-to-Multipoint (PtM) using the smartBridges airPoint™ Nexus and airClient™ Nexus radios. The smartBridges airPoint can support as many as 128 clients.
Selecting the right kind of Wi-Fi equipment is mission critical, especially for example, certain ventures like e-Governance. Below are some suggested specifications to consider when selecting the equipment for a reliable network implementation.
The figure below shows the basic anatomy of the suggested Wi-Fi equipment.
Before we go about calculating and planning a link let us get familiar with some terms that will help us understand the basis of link planning.
Terms and Concepts
Gain
Gain is the term used to describe an increase in an RF signal's amplitude. Gain is usually an active process meaning that an external power source, such as an RF amplifier, is used to amplify the signal or that a high-gain antenna is used to focus the beam width of the signal to increase its amplitude. Typically, more power is better, but there are cases, such as when a transmitter is radiating power very close to the legal power output limit, where added power would be a serious problem. Also note that a very high power may take the device in saturation which can result in the bandwidth going down.
There are three generic categories of RF antennas:
Gain can be in terms of the transmitting power of the device (dBm) or the gain of the antenna (dBi). Thus the gain of the antenna and the transmitting device put together can result into the total effective power.
Double/Half power ------------->>>>>> Add/Subtract 3dB
Ten Times/One-tenth Power --------------->>>>>> Add/Subtract 10dB
Free Space Loss
As signals spread out from a radiating source, the energy is spread out over a larger surface area. As this occurs, the strength of that signal gets weaker. Free space loss (FSL), measured in dB, specifies how much the signal has weakened over a given distance.
FSL = 36.6+20 logF+20log D
Where F: Frequency in MHz and D: Distance in miles
Distance 2 Miles 4 Miles 6 Miles 10 Miles 20 Miles
2.4 GHz 110 dB 116 dB 119 dB 124 dB 130 dB
FSL:
5.8 GHz 118 dB 124 dB 127 dB 132 dB 138 dB
FSL:
Add/subtract 6dB ------------->>>>>> Double/half the distance
Fresnel Zone
The area that the signal spreads out into is called the Fresnel zone. An obstacle in the Fresnel zone diffracts or bends part of the radio signal away from the straight-line path. On a PtP radio link, this refraction reduces the amount of RF energy reaching the receiving antenna.
A consideration when planning or troubleshooting an RF link is the Fresnel Zone. The Fresnel Zone occupies a series of concentric ellipsoid shaped areas around the LOS path, as can be seen in the figure. The Fresnel Zone is important to the integrity of the RF link because it defines an area around the LOS that can introduce RF signal interference if blocked. Objects in the Fresnel Zone such as trees, hills and buildings can diffract or reflect the main signal away from the receiver, changing the RF LOS. These same objects can absorb or scatter the main RF signal, causing degradation or complete signal loss.
Source: CWNA Guide
The radius of the Fresnel Zone at its widest point can be calculated by the following formula:
where d is the link distance in miles, f is the frequency in GHz and r is in feet.
For example, suppose there is a 2.4 GHz link 5 miles (8.35 km) in length. The resulting Fresnel Zone would have a radius of 31.25 feet (9.52 meters).
A question commonly asked about the Fresnel Zone when using indoor wireless LAN equipment such as PC cards and access points is how blockage of the Fresnel Zone affects indoor installations. In most indoor installations, RF signals pass through, reflect off, and refract around walls, furniture, and other obstructions. The Fresnel Zone is not encroached upon unless the signal is partially or fully blocked. This is sometimes the case, but is rarely noticed due to most wireless users being mobile. In a mobile environment, the Fresnel Zone is constantly changing so the user normally dismisses it thinking that the coverage is simply "bad" where they are located - giving no thought to why the coverage is bad.
Receive Signal Level
Receive signal level is the actual received signal level (usually measured in negative dBm) presented to the antenna port of a radio receiver from a remote transmitter.
For a wireless ISP (WISP) to provide reliable service to a customer (or client), the following two conditions must be present:
1. Sufficient transmit power to reach the receiver (or client) at the far end of the link, and
2. The power that reaches the far-end receiver is louder than the noise that reaches the receiver. In other words, the signal-to-noise ratio (SNR) must be positive – the signal stronger than the noise.
Before calculating the link budget, we need to understand the difference in the way power is lost when it travels through a wired link (for example, an Ethernet cable) compared to the way power is lost when it travels through a wireless link.
Power in a wire is lost “linearly”. When the length of a wired link is doubled, the power loss is doubled. This occurs because the size of the wire and therefore the area of the wire (the cross-sectional area) remain constant from the beginning to the end of the wire. As the signal power travels forward through the wire, it only expands in one direction - along the length of the wire.
In contrast to the wired link, the power traveling across a wireless link expands in all directions. Like a flashlight shining on the inside of a bowl, the wireless power spreads out left and right and up and down as it travels forward. The further the wireless power travels, the more it spreads out, the larger the area it travels through, and the quicker the power level decreases. Because the wireless power spreads out in all directions, it decreases much faster than linearly - it decreases logarithmically, according to the “inverse-square” law.
The following graph compares the power lost in a wired link versus the power lost in a wireless link when the link length is extended. Imagine starting with links that are one mile long and then increasing them to 2, 3, 4 and 5 miles.
Percentage of Remaining Energy When Extending the Link Distance
Start with Link distance Link distance Link distance Link distance
1-mile links = 2 miles = 3 miles = 4 miles =5miles
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Linear Power in 1/2 1/3 1/4 1/5
Wired link
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Inverse Power 1/4 1/9 1/16 1/25
Wireless link
Transmit Power and Receiver Sensitivity
Receiver sensitivity is the weakest RF signal level (usually measured in negative dBm) that a radio needs receive in order to demodulate and decode a packet of data without errors. The transmit power is the RF power coming out of the antenna port of a transmitter. It is measured in dBm, Watts or milliWatts and does not include the signal loss of the coax cable or the gain of the antenna.
* Transmit power and receiver sensitivity are expressed relative to a reference level of 1 milli-Watt (mW) and abbreviated dBm.
The receiver sensitivity of a good ISM band receiver ranges from 75 dBm to -90 dBm. -90 dBm means the receiver can decode a signal at 1 nanoWatt.
Receiver sensitivity is generally measured by reducing the input power until the error level exceeds a defined threshold. It is common to indicate the sensitivity as the level when the error rate has increased to 10E-6 (one bit error per 1 million bits of data). With a lower data rate, the connection will be more robust. Typically, the sensitivity decreases by 3dB when the data rate is doubled.
The EIRP is limited by the regulatory authority.
Effective Isotropic Radiated Power
Effective isotropic radiated power (EIRP) is the actual RF power as measured in the main lobe (or focal point) of an antenna. It is equal to the sum of the transmit power into the antenna (in dBm) added to the dBi gain of the antenna. Since it is a power level, the result is measured in dBm.
EIRP is the power actually radiated by the antenna element, as shown in the figure. This concept is important because it is regulated by the governing body in the country of operation and because it is used in calculating whether or not a wireless link is viable. EIRP takes into account the gain of the antenna.
EIRP = TX Power – Coaxial Cable Loss + TX Antenna Gain
Source: CWNA Guide
Suppose a transmitting station uses a 10-dBi antenna (which amplifies the signal 10-fold) and is fed by 100 milliwatts from the intentional radiator. The EIRP is 1000 mW, or 1 Watt. The FCC has rules defining both the power output at the intentional radiator and the antenna element.
System Operating Margin/Fade margin
System operating margin (SOM) is the difference (measured in dB) between the nominal signal level received at one end of a radio link and the signal level required by that radio to assure that a packet of data is decoded without error.
RX Signal = EIRP – FSL + RX Antenna Gain – Coax Cable Loss
Fade margin/SOM =RX Received signal – Receiver Sensitivity
Ideally the fade margin should be more than 20 dB. Less SOM can result into unstable link.
Refer here or here for the wireless link calculator.
Calculating the link
Link calculation can be done to calculate the Fade margin or the Link Distance. Let’s consider an example. The image below is of a Link Budget calculator. To calculate the Link Distance and the Fade Margin one must have first hand information of the following:
* Transmitting power of the wireless equipment
* Antenna Gain
* Receiver Sensitivity (should be always more than -75dBm)
* Cable Loss
* EIRP (Usually set the by the regulatory authorities in the country of operation)
The above image is a screenshot of the smartBridges Link Budget calculator.
The GPS calculator shown helps us to calculate the distance based on the latitude, longitude of the host and the remote device.
Consider an example now where the link budget calculations can be used derived on the following formulae:
* EIRP = TX Power – Coaxial Cable Loss + TX Antenna Gain
* RX Signal = EIRP – FSL + RX Antenna Gain – Coax Cable Loss
* Fade margin =Received signal – Receiver Sensitivity
* The intended distance of the link is fixed to15 kms
* The antenna gain is set to 23dbi
* The output transmitting power of the Nexus™ product is 13dBm
* In this case the Receiver sensitivity is set to -89dBm as the throughput to be achieved is 5.5 mbps (data rate of 9Mbps) for the Nexus™ product
* This as per the calculations gives us the fade margin to be around 17 dB
However, before we fix on the fade margin or the distance we must also consider the Fresnel Zone calculations.
Source: Plassey Broadband Wireless
The above chart exhibits the basis of defining a Fresnel Zone and its interference with the earth’s curvature. Assuming the earth is curved at the given distance in a link. Therefore the Fresnel Zone has to be calculated in such a way so that it does not interfere with the earth’s curvature. This can be done by adjusting the antenna’s height so that proper LOS can be maintained. Considering the importance of Fresnel Zone clearance, it is also important to quantify the degree to which the Fresnel Zone can be blocked. Since an RF signal, when partially blocked, will bend around the obstacle to some degree, some blockage of the Fresnel Zone can occur without significant link disruption. Typically, 20% - 40% Fresnel Zone blockage introduces little to no interference into the link. It is always suggested to err on the conservative side allowing no more than 20% blockage of the Fresnel Zone. Obviously, if trees or other growing objects are the source of the blockage, you might want to consider designing the link based on 0% blockage.
Effect of Adverse Weather Conditions
It is important to research any unusual weather conditions that are common to the site location. These conditions can include excessive amounts of rain or fog, wind velocity, or extreme temperature ranges. If extreme conditions exist that may affect the integrity of the radio link, it is recommended that these conditions be taken into consideration early in the planning process.
Rain and Fog
Except in extreme conditions, attenuation (weakening of the signal) due to rain does not require serious consideration for frequencies up to the range of 6 or 8 GHz. When frequencies are at 11 or 12 GHz or above, attenuation due to rain becomes much more of a concern, especially in areas where rainfall is of high density and long duration. If this is the case, shorter paths may be required.
In most cases, the effects of fog are considered to be much the same as rain. However, fog can adversely affect the radio link when it is accompanied by atmospheric conditions such as temperature inversion, or very still air accompanied by stratification. Temperature inversion can negate clearances, and still air along with stratification can cause severe refractive or reflective conditions, with unpredictable results. Temperature inversions and stratification can also cause ducting, which may increase the potential for interference between systems that do not normally interfere with each other. Where these conditions exist, it is recommended to have shorter paths and adequate clearances.
Atmospheric Absorption
A relatively small effect on the link is from oxygen and water vapor. It is usually significant only on longer paths and particular frequencies. Attenuation in the 2 to 14 GHz frequency range, which is approximately 0.01 dB/mile, is not very significant.
Wind
Any system components mounted outdoors will be subject to the effect of wind. It is important to know the direction and velocity of the wind common to the site. Antennas and their supporting structures must be able to prevent these forces from affecting the antenna or causing damage to the building or tower on which the components are mounted. Antenna designs react differently to wind forces, depending on the area presented to the wind. This is known as wind loading. Most antenna manufacturers will specify wind loading for each type of antenna manufactured.
Lightning
The potential for lightning damage to radio equipment should always be considered when planning a wireless link. A variety of lightning protection and grounding devices are available for use on buildings, towers, antennas, cables, and equipment, whether located inside or outside the site that could be damaged by a lightning strike. Lightning protection requirements are based on the exposure at the site, the cost of link down-time, and local building and electrical codes. If the link is critical, and the site is in an active lightning area, attention to thorough lightning protection and grounding is critical.
Lightning Protection
To provide effective lightning protection, install antennas in locations that are unlikely to receive direct lightning strikes, or install lightning rods to protect antennas from direct strikes. Make sure that cables and equipment are properly grounded to provide low-impedance paths for lightning currents. Install surge suppressors on telephone lines and power lines. Lightning protection is recommended for both coaxial and control cables leading to the wireless device. The lightning protection should be placed at points close to where the cable passes through the bulkhead into the building, as well as near the wireless device. All smartBridges products include grounding wires, so please make sure that the antenna is properly grounded.
Improving Coverage and Throughput
* Select data rate according to the actual utilization; with lower data rate allows longer distances to be achieved.
* Selection of Autofall back mode.
* Keeping the transmit power of equipment low and using a higher gain antenna will improve the data rate and coverage.
* Selecting equipment with RF interference mitigation capabilities
* Selecting equipment that takes care of multi-path issues
* Using MAC based authentication only for security and disabling a WEP/WPA/AES if higher end security standards are not required. This will improve the throughput.
* Selection of sector can improve the range of coverage in a particular direction
Conclusion
Before selecting the link, it is important to first know a few parameters such as the gain of the antenna, transmitting power of the device, receiver sensitivity and the permissible maximum EIRP. Fresnel zone calculations should also be taken into consideration. Using a simple GPS calculator the distance based on the latitude and the longitude of the host and the remote device can be calculated. Thus by proper selection of various parameters a proper stable link under abnormal circumstances can also be obtained.
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